Viscoelastic properties of tissues are closely related to the tissue state and pathology. Vibro-acoustography (VA) is new imaging method developed in our laboratory that produces a display of the dynamic response of tissue to a vibrating force at a selected low frequency (kHz range). Such images may be used for evaluation of tissue pathology. A vibro-acoustography system uses the radiation force of ultrasound to produce a highly concentrated oscillating force that vibrates the object at each point. Object response is recorded by a hydrophone and used to produce the image. The result is a speckle-free, transverse view, representing the dynamic response of the object to the vibrating force. In preliminary in vitro experiments, we have shown that vibro-acoustography can be used to image hard materials such as calcifications inside human arteries, very small particles such as micron-size microcalcifications in human breast tissue, and lesions in liver. VA can also be used to image and evaluate the integrity of prosthetic heart valves. The purpose of this proposal is to develop a vibro-acoustogrraphy system with a contact probe for in vivo applications. The goal of this research will be achieved in two phases. The first phase consists of computer simulation of the system. The second phase focuses on system implementation. The aims of the first phase include: system design, modeling, and evaluating system performance. The aims of the second phase include design and construction of the ultrasound probe, design and construction of system electronics (transmitter, receiver, and monitor), and evaluating and optimizing system performance. Successful completion of this research will result in a new class of noninvasive imaging and tissue evaluation device, which will provide information about the mechanical properties of tissue that are not available from other imaging modalities.